1. Field of the Invention
The present invention relates to a zoom lens system, and particularly to a compact zoom lens system with a short overall length and a high image resolution for forming a real image on a digital or non-digital image pickup device of a camera.
2. Description of Prior Art
In recent years, with the rapid integration of optical technology into digital electronic technology, most electronic devices such as camera mobile phones, small digital cameras and small video cameras have been integrated with a zoom lens system for effecting the photographic function. Such a zoom lens system is generally required to be small in size and light in weight for portability, while being capable of providing a high zoom ratio and a high image resolution. Generally, a high zoom ratio lens system consists of a plurality of lens groups and a considerable number of constituent lenses, whereby the overall length of the lens system is rather long. To meet the additional high-resolution requirement, special low dispersion lenses and aspheric lenses are further included in the lens system.
With the development of semiconductor technology, aspheric lenses have been widely used in a photographic lens system. The adoption of an aspheric lens effectively corrects spherical aberration associated with spherical lenses. Further, an aspheric lens functions equivalent to several spherical lenses, whereby the cost can be reduced and a compact lens system can be ensured. For a zoom lens system, in order to obtain the desired zoom ratio and optical performance within the entire zoom range while ensuring a compact configuration, the lens configuration for each lens group of the zoom lens system must be carefully designed. Conventionally, a photographic zoom lens system generally employs three lens groups in a negative-positive-positive refractive power configuration, wherein two of the three lens groups are movable for realizing zooming. However, the movement ranges of the two movable lens groups are relatively large, and the imaging performance dramatically varies with the increase of zoom ratio. To overcome these disadvantages, a zoom lens system consisting of four lens groups and thus an increased number of constituent lenses has been introduced. Three of the four lens groups are generally configured to be movable for realizing zooming. However, in order to achieve a high zoom ratio of 5× and a high image resolution, a long overall length of such a zoom lens system is necessitated for allowing movement of the three lens groups. When all the four lens groups are configured to be movable, both the number of constituent lenses and the manufacturing cost of the zoom lens system are generally increased. Further, whether three or four lens groups are configured to be movable for effecting zooming, the outer diameter of each lens group of the conventional four-group zoom lens system is generally large. This results in a large retraction space and a large retraction length when the conventional four-group zoom lens system is retracted.
It is known in the zoom lens art that, to reduce the overall length of a zoom lens system, a direct and efficient solution is to reduce the movement range of each lens group during zooming. Unfortunately, this solution generally requires an increase in the manufacturing precision and difficulty of the zoom lens system and a reliable aberration correction effect is also difficult to be ensured. Therefore, how to simplify the lens configuration so as to reduce the size and weight of the whole zoom lens system while maintaining high optical performance including a high zoom ratio and a high image resolution is a difficult problem encountered by a zoom lens designer.
Hence, an improved four-group zoom lens system, which is compact in size and short in retraction length while providing a high zoom ratio and a high image resolution, is desired to overcome the above problems encountered in the prior art.